1. Field of the Invention
The present invention relates to a mobile communications system including mobile communications terminal equipment capable of establishing communication with any one of a plurality of communications systems that provide respective cells that are overlapped, e.g., either of a ground (i.e., terrestrial) cellular communications system and a satellite mobile communications system, the mobile communications terminal equipment providing and using information on a communications channel selected when switching between the plurality of communications systems.
2. Description of the Prior Art
Referring next to FIG. 10, there is illustrated a block diagram showing the structure of a prior art mobile communications system in which mobile communications terminal equipment can establish connection with any one of a plurality of mobile communications systems. In the figure, reference numeral 1 denotes a base station for use with a mobile communications system A, numeral 2 denotes a cell provided by each base station 1, numeral 3 denotes a communications satellite for use with a mobile communications system B, and numeral 4 denotes a cell provided by the communications satellite 3. In general, the communications satellite 3 can provide a plurality of cells 4. Reference numeral 5 denotes mobile communications terminal equipment capable of establishing connection with either of the mobile communications systems A and B by switching between them, and numeral 6 denotes a control channel through which each of the base station 1 and the communications satellite 3 can transmit a variety of pieces of information to the mobile communications terminal equipment.
Next, a description will be made as to the operation of the prior art mobile communications system. Each of the base station 1 and the communications satellite 3 controls communications with the mobile communications terminal equipment 5 using the control channel thereof. Each of the base station 1 and the communications satellite 3 broadcasts information on control channels used by each respective mobile communications system, such as information on the frequencies of control channels used by neighboring cells which constitute a service area and the size of cells provided by each base station.
In each mobile communications system, the mobile communications terminal equipment 5 can switch between cells by changing channel parameters from the current use of transmission and reception channels to suitable channels selected for a neighboring cell based on the control channel information transmitted via the control channel and the radio condition of each neighboring cell, and complete the switching operation by performing a process of registering its location or the like.
On the other hand, when switching between the two mobile communications systems, the mobile communications terminal equipment 5 itself has to search for an available channel from all control channels defined within the new communications system to use because no control channel information about neighboring service cells covered by the new communications system is provided by the current communications system being used by the terminal. As a result, in most cases the user has to wait a few tens of seconds until connection with the new communications system is established. To be more specific, when switching between two mobile communications systems, the mobile communications terminal equipment 5 tries to establish connection with the new communications system via each of available channels one after another. The mobile communications terminal equipment 5 then evaluates the reception level and so on of each available channel, and repeats a process of searching for a channel with good receiving signal quality based on the evaluation result. The process is needed to establish stable communication between the mobile communications terminal equipment 5 and the new mobile communications system. In general, since there are many available channels to be evaluated, much time is spent in evaluating the reception levels of all available channels. Thus the mobile communications terminal equipment 5 cannot use the new mobile communications system during the evaluation of the reception levels of all available channels.
To overcome the above problem, it is conceivable that the mobile communications terminal equipment 5 pre-stores the correspondence of control channel information between all communications systems, so that the mobile communications terminal equipment 5 can switch to a new mobile communications system by receiving the corresponding channel of the new mobile communications system. However, in the case that a communications satellite in the mobile communications system B of FIG. 10 is not a geostationary satellite, each of the cells provided by the communications satellite moves with time even though the mobile communications terminal equipment 5 is stationary. Thus, the correspondence of control channel information cannot be obtained.
The present invention is made to overcome the above problem. It is therefore an object of the present invention to provide a mobile communications system in which mobile communications terminal equipment, which can establish communication with two or more mobile communications systems, can perform a process of switching between two mobile communications systems in a short time.
In accordance with one aspect of the present invention, there is provided a mobile communications system including mobile communications terminal equipment capable of establishing communication with two or more different mobile communications systems including the mobile communications system by switching among the mobile communications systems, and at least a base station that can establish communication with the mobile communications terminal equipment, the base station including a transmitting unit for transmitting channel information on the other mobile communications systems to the mobile communications terminal equipment, and the mobile communications terminal equipment including a channel information storage unit for receiving and storing the channel information on the other mobile communications systems from the base station, the mobile communications terminal equipment being able to establish connection with one of the other mobile communications systems based on the channel information when needed. Accordingly, the mobile communications terminal equipment can speedily switch to another mobile communications system based on the received channel information for the other communications systems.
In accordance with a preferred embodiment of the present invention, the base station includes a channel information storage unit for storing the channel information for the other mobile communications systems, the storage unit extracting the channel information for each of a plurality of cells provided by the base station so that cells illuminated by extracted corresponding channels cover each of the plurality of cells and the vicinity of each of the plurality of cells, from the channel information on the other mobile communications systems, and storing the extracted channel information therein. Accordingly, when switching to another mobile communications system, the mobile communications terminal equipment can reduce the time required for performing a communications channel searching operation.
In accordance with another preferred embodiment of the present invention, the mobile communications terminal equipment includes a unit for assigning priorities to the other mobile communications systems other than the mobile communications system. When switching from the mobile communications system to another mobile communications system, the mobile communications terminal equipment establishes connection with one of the other mobile communications systems according to their priorities. Accordingly, the usefulness of the mobile communications terminal equipment is enhanced when switching among a number of communications systems.
In accordance with another preferred embodiment of the present invention, the channel information is a set of cell numbers assigned to cells provided by the other mobile communications systems. The mobile communications terminal equipment includes a channel parameter table including, as each element, a cell number and a set of corresponding channel parameters, and can establish connection with one of the other mobile communications systems by retrieving a set of channel parameters from the channel parameter table based on the channel information. Accordingly, the structure of the system can be simplified because each of the plurality of cell numbers transmitted from the base station to the mobile communications terminal equipment represents corresponding channel information.
In accordance with another aspect of the present invention, there is provided a ground cellular communications system including a mobile communications terminal equipment capable of switching between the ground cellular communications system and either a mobile satellite communications system or another ground cellular communications system to establish communication with one of them, and at least a base station for use with the ground cellular communications system, with which the mobile communications terminal equipment can establish communication, the base station including a cell location storage unit for storing the locations of cells provided by the mobile satellite communications system and the other ground cellular communications system, a cell extraction unit for extracting some cells located either within a cell provided by the ground cellular communications system or in the vicinity of the cell from all the cells provided by the mobile satellite communications system and the other ground cellular communications system based on the cell locations stored in the cell location storage unit, and a channel information transmitting unit for transmitting channel information on the cells extracted by the cell extraction unit to the mobile communications terminal equipment, and the mobile communications terminal equipment including a channel information storage unit for receiving and storing the channel information from the base station, the mobile communications terminal equipment establishing connection with either the mobile satellite communications system or the other ground cellular communications system based on the channel information stored in the channel information storage unit thereof when needed. Accordingly, the mobile communications terminal equipment can speedily switch to either the mobile satellite communications system or the other ground cellular communications system based on the channel information on the mobile satellite communications system and the other ground cellular communications system, which is provided by the ground cellular communications system.
In accordance with a preferred embodiment of the present invention, the cell location storage unit computes the locations of cells provided by the mobile satellite communications system based on orbit information on one or more Earth-orbiting satellites, and stores the cell locations therein. Accordingly, the cell location information transmitted from the mobile satellite communications system to the ground cellular communications system can be simplified.
In accordance with a further aspect of the present invention, there is provided a mobile satellite communications system including a mobile communications terminal equipment capable of switching between the mobile satellite communications system and either a ground cellular communications system or another mobile satellite communications system to establish communication with one of them, and at least a base station for use with the mobile satellite communications system, with which the mobile communications terminal equipment can establish communication, the base station including a cell location storage unit for storing the locations of cells provided by the ground cellular communications system, a cell extraction unit for extracting some cells located either within a cell provided by the mobile satellite communications system or in the vicinity of the cell from all the cells provided by the ground cellular communications system and the other mobile satellite communications system based on the cell locations stored in the cell location storage unit, and a channel information transmitting unit for transmitting channel information on the cells extracted by the cell extraction unit to the mobile communications terminal equipment, and the mobile communications terminal equipment including a channel information storage unit for receiving and storing the channel information from the base station, the mobile communications terminal equipment establishing connection with either the ground cellular communications system or the other mobile satellite communications system based on the channel information stored in the channel information storage unit thereof when needed. Accordingly, the mobile communications terminal equipment can speedily switch to either the ground cellular communications system or the other mobile satellite communications system based on the channel information on the ground cellular communications system and the other mobile satellite communications system, which is provided by the mobile satellite communications system.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.